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Electronically Reconfigurable Liquid Crystal Based Mm-WavePolarization Converter

Doumanis, E., Goussetis, G., Dickie, R., Cahill, R., Baine, P., Bain, M., Fusco, V., Encinar, J. A., & Toso, G.(2014). Electronically Reconfigurable Liquid Crystal Based Mm-Wave Polarization Converter. IEEE Transactionson Antennas and Propagation, 62(4), 2302-2307. https://doi.org/10.1109/TAP.2014.2302844

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Electronically Reconfigurable Liquid Crystal

Based Mm-wave Polarization Converter

E. Doumanis, Member, IEEE, G. Goussetis, Senior

Member, IEEE, R. Dickie, R. Cahill Senior Member,

IEEE, P. Baine, M. Bain, V. Fusco, Fellow IEEE, J. A.

Encinar, Fellow IEEE, G. Toso Senior Member, IEEE

Abstract—An electronically tunable reflection polarizer which

exploits the dielectric anisotropy of nematic liquid crystals (LC)

has been designed, fabricated and measured in a frequency band

centered at 130 GHz. The phase agile polarizing mirror converts

an incident slant 45º signal upon reflection to Right Hand

Circular (RHCP), orthogonal linear (-45o) or Left Hand Circular

(LHCP) polarization depending on the value of the voltage

biasing the LC mixture. In the experimental set-up this is

achieved by applying a low frequency bias voltage of 0 V, 40 V

and 89 V respectively, across the cavity containing the LC

material.

Index Terms—mm-wave, submm-wave, tunable, polarizer,

liquid crystals, space communications, imaging, remote sensing,

earth observation, polarimetric systems, interferometry.

I. INTRODUCTION

atellite polarimetric imaging systems, such as those used

for passive and active Earth observation to measure the

surface wind vector from space [1, 2] and vegetation

properties [3], commonly involve dedicated receive and

transmit chains for each polarization state. Tunable polarizers

can reduce redundancy, volume/mass budget and cost.

Dynamic polarization agility is also desirable in radar

applications for defense and remote sensing to enhance

detection and measurement of a feature in a radar scene [4] as

well as wireless and satellite telecommunications to minimize

feed losses and polarization purity impairments. Polarization

agility in quasi-optical mm-wave systems can also be used to

create tunable isolators (switches) [5] as well as frequency

tunable interferometers for filtering and diplexing [6].

Traditional tunable polarizer technology relies on

mechanical motors or rotors [4] leading to increased energy

consumption and mass as well as compromised reliability. In

This Manuscript received August 29, 2013. This work was supported by

the European Space Agency, under project 4000103061. G. Goussetis would like to acknowledge support by FP7 project DORADA (IAPP-2013-610691).

E. Doumanis is with Bell-Labs Alcatel-Lucent, Blunchardstown Industrial

Park, Dublin 15, Ireland (e-mail: [email protected]) G. Goussetis is with the School of Engineering and Physical Sciences,

Heriot-Watt University, Edinburgh EH14 4AS, UK (e-mail:

[email protected]) R. Dickie, R. Cahill, P. Baine, M. Bain and V. Fusco are with the Institute

of Electronics, Communications and Information Technology, Queen’s

University Belfast, Belfast BT3 9DT, Northern Ireland, UK, (e-mail: [email protected])

J. A. Encinar is with the Department of Electromagnetism and Circuit

Theory, Universidad Politécnica de Madrid, E-28040, Madrid, Spain (e-mail:

[email protected]).

G. Toso is with the European Space Agency ESA–ESTEC, 2200 AG

Noordwijk, The Netherlands, (e–mail: [email protected]).

order to address such limitations, integrated solutions based on

e.g. MEMS [7] and piezoelectric ultrasonic motors [8] have

been proposed. Despite their compact physical dimensions,

these technologies are suitable for switched-based

architectures offering discrete polarization states. Moreover,

such technologies are mostly relevant to waveguide-based

polarizers which are difficult to scale to (sub)mm-wave

frequencies; free-space polarizers would typically require

multiple actuators which is undesirable for practical

implementation.

Fig. 1. Schematic of the proposed reconfigurable polarizer that exploits a

tunable Liquid Crystal substrate biased by a low frequency quasi-static source.

Recently the use of nematic liquid crystals as a dielectric

with electronically tunable permittivity has emerged as a

technology suitable for continuous tuning of (sub)mm-wave

and THz devices [9]. By applying an external field dynamic

adjustment of the effective dielectric properties can be

achieved [10], [11]. A waveguide-based liquid crystal

polarizer was proposed in [12] as means to deliver continuous

tuning of the polarization state between -90o to 90

o and was

demonstrated at Q–band. Significantly, the cost and

complexity of integrating liquid crystals in multi-wavelength

free-space devices does not increase proportionally with the

size; this integration is also compatible with mature

semiconductor technologies hence enabling scaling to the

(sub)mm-wave range. Liquid crystals have therefore been

successfully employed in the development of tunable

frequency selective surfaces [13]. They have also been

employed in the development of tunable reflectarray cells for

sum and difference monopulse radiation patterns [14] and

dynamic beam steering at W-band [15].

In this communication we present a mm-wave tunable

quasi-optical reflection polarizer that exploits liquid crystal

technology to dynamically change the polarization state of an

incoming wave. In particular we demonstrate the first reported

free-space polarizer operating at frequencies larger than 100

GHz with the capability to convert a linearly polarized

incoming wave to a reflected wave with a polarization state

that can be selected to be RHCP, LHCP or the orthogonal

linear polarization, thus covering an entire meridian on the

Poincare sphere and, when considering the polarization of the

incident signal, covers the four key canonical polarization

S

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/TAP.2014.2302844

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states. The polarizer design is based on a recently proposed

class of engineered mirrors [16]. A static W-band prototype

has been presented in [17]. This feature is exploited here in

conjunction with the tunable dielectric permittivity tensor of a

liquid crystal mixture in order to develop an electronically

reconfigurable reflection polarizer. A schematic of the

structure is depicted in Fig 1.

II. RECONFIGURABLE MM-WAVE POLARIZER

A. Design and realization

The operation of the reconfigurable polarizer is

demonstrated by means of a structure designed to operate at

130 GHz. Simulations of the electromagnetic scattering have

been performed using the commercial electromagnetic solver

CST [18]. Fig. 2 depicts the geometry and dimensions of a

unit cell and a crossectional view of the device. The periodic

array of dipoles is printed on a 100 mm diameter metal plated

quartz substrate with dielectric permittivity εr = 3.78 and loss

tangent tan δ= 0.0004. The thickness of the metallized pattern

is 3μm. In order to reduce the possibility of breakage during

processing, the thickness of the quartz substrate is chosen to

be 250 μm and a metalized silicon wafer is used to form the

ground plane. A thin (100 nm) resistive boron diffused

polycrystaline silicon film [19] was deposited by a low

pressure Chemical Vapour Deposition process at 620oC on the

un-patterned side of the quartz wafer to give an almost mm-

wave transparent but low frequency conductive layer. This

layer is used for the establishment of a low frequency quasi-

static electric field in the cavity to bias the liquid crystals; it

has been experimentally determined that a low frequency field

is best suitable for the biasing of the LC [13]. The average

measured sheet resistance was 28kΩ/ and therefore

this layer

does not interact strongly (<0.2 dB transmission loss))with the

incoming mm-wave signal.

(a) (b)

Fig. 2. Schematic of unit cell (a) top view and (b) cross-section of the

polarizer. The circles depict the microsphere spacers that define the thickness of the cavity in which the liquid crystal mixture is encapsulated. Dimensions

(in mm): l=0.6, w=0.04, dx=0.8, dy =1.0, quartz thickness (t) = 0.25, cavity

thickness (d) =0.05.

In order to align the direction of the LC molecules parallel

to the cell substrate, an alignment layer consisting of a 1 μm

thick polyimide film was deposited on the two metalized

surfaces and subsequently rubbed with velvet to form

microgrooves. The two wafers were separated by a 50 m

gap, which was created by placing Micropearl SP precision

glass spheres [20] around the outer region of the device, and

bonded using slow drying glue, thus forming a cavity where

the LC mixture is hosted. The construction of the polarizer

was completed by applying a thin film of epoxy resin around

the edge of the stacked wafers to seal the cavity leaving a

small filling port for introducing the LC material. A vacuum

filling technique was employed to insert the liquid crystals

into the 50 µm thick cavity via the filling port; the elongated

LC molecules align along the groves of this rubbing layer

during filling. The cavity was subsequently sealed and two

small copper to aluminium contacts were attached on the

outside of the cell using silver conductive glue to enable

biasing. For the construction of this prototype device we have

used GT3-23001, which is a custom LC mixture developed for

microwave applications by Merck and characterised at mm

wavelengths [19].

(a)

(b)

Fig. 3. Simulated reflection (a) phases and (b) magnitudes obtained with

CST for TE and TM incident waves incident on the LC based polarizer shown in Fig. 2 at angle θ= 45o. The tensors shown in eq. (1) have been used for the

simulation of the unbiased (OFF) and biased (ON) states, where ,

, , . Measured reflection phases and

amplitudes for various biasing voltages are superimposed.

The rubbing layers as well as the microsphere spacers have

negligible effect on the electromagnetic response of the

structure and therefore have not been included in simulation.

Likewise dipole thickness has been ignored as it was shown



that the performance for metal features of the order of 3 μm is

almost identical to that for 2D flat elements. As described in

[21], the dielectric properties of the liquid crystal mixture are

best characterized by a permittivity tensor that takes into

account the orientation of the long molecules of the mixture.

Assuming that the molecules are aligned along the y-axis in

the off state (unbiased) and along the z-axis in the on state

(biased) (Fig. 2), then the complex dielectric permittivities in

those two cases are:

(

) (

) (1)

Similar tensors describe the anisotropic loss tangent and are

not presented here for brevity.

Based on these parameters and the aforementioned substrate

stack, the dimensions of the dipole array were optimized using

the design methodology described in [16]. The incidence

angle, marked as θ in Fig. 1, is 45o. For the purposes of the

design we have used the dielectric properties for GT3-23001

measured in the range 140 – 165 GHz [19]: ,

, , .

The primary design aim is to enable the polarization of the

reflected waves to vary between RHCP and LHCP at the two

extreme values of the permittivity tensor for zero, , and

fully biased liquid crystals, . With reference to the

schematic shown in Fig. 1, the reflection properties of the TM

polarized waves are negligibly affected by the dielectric

properties of the liquid crystal; therefore the required range of

reflection coefficient phases for TE polarized waves can be

obtained. Given that between these two values, the

polarization of the reflected waves moves continuously across

an entire meridian of the Poincare sphere, linear polarized

waves which are orientated orthogonal to the direction of the

incidence signals will occur across this continuum; this will

occur at the frequency where the phase difference between the

TE and TM reflected field is 180º.

A further design consideration relates to the thermal losses.

Structures such as the one shown in Fig. 1 are known to

exhibit increased thermal losses around the frequency where

the reflection phase is zero. This is due to a resonance formed

in the cavity between the periodic array and the ground plane

and is also known as the Artificial Magnetic Conductor

(AMC) frequency [22]. As a general observation, array

designs which provide a large change in the TE reflection

phases for a given set of and tensors, are also

unavoidably subjected to increased losses near the resonance.

A trade-off between the tuning range of the TE reflection

phase and insertion loss is therefore required during the design

stage.

The optimized dimensions for the dipole array are provided

in the legend of Fig. 2 and the simulated reflection phase and

magnitude coefficients of the polarizer in the unbiased

( and fully biased ( states of the liquid crystal are

depicted in Fig. 3. It is noted that in the phase responses

shown in Fig. 3 the frequency of the reflection minimum

observed in Fig. 3b does not coincide with the zero reflection

phase (Fig. 3a), as is commonly the case in AMCs [22];

instead an additional phase delay of approximately 90o is

introduced as a result of propagation within the substrate. This

is attributed to the choice of the reference plane. It is noted

that the reflection minimum still coincides with the maximum

group delay (steepest reflection phase slope) as expected. The

choice of the reference plane further explains the reflection

phase values observed for TM polarized waves.

As shown in Fig. 3a, the TM reflection phase is scarcely

affected by the biasing of the liquid crystal but for TE the

reflection phase varies across a range of about 184o

at 130

GHz. The cavity resonance associated with the zero reflection

phase [22] is evident in Fig. 3b where the TE reflection

coefficient is shown to be lower than the value predicted for

the TM plane, peaking at -6 dB and -7 dB for the unbiased and

biased case at the AMC frequency respectively. The relatively

high level of losses for TE incidence waves can be understood

in view of the discussion in reference [9], where it was

elaborated that larger variations of the reflection phase for this

class of devices is associated with increased losses.

Fig. 4. Simulated axial ratio and reflection magnitudes (obtained with

CST) for incoming plane waves at angle θ= 45o linearly polarized at angle ξ=

45o and ξ= 58o on the unbiased and biased LC based polarizer of Fig. 2. Inset shows axial ratio curves zoomed in the vicinity of the operating frequency.

Using the predicted responses shown in Fig. 3, the axial

ratio (AR) of the reflected waves based on an incoming

linearly polarized signal at slant angle ξ= 45o (as defined in

Fig. 1) was calculated for both the unbiased and biased states

and is plotted in Fig. 4. For each state the AR curve exhibits

two minima, which correspond to RHCP and LHCP modes,

and a maximum, which corresponds to the orthogonal linear

polarization. In particular, Fig. 4 shows that when the

polarizer is biased, the AR curve shifts towards lower

frequencies. For the unbiased case the phase difference

between TE and TM waves at 130 GHz is 90o indicating that

reflection occurs in RHCP. For the biased case the next AR

minimum, which corresponds to a phase difference between



TE and TM waves of 270o

and LHCP polarization state,

occurs at 128.5 GHz; although in principle it would be

sufficient for the theoretical design to produce a frequency

shift of the AR curve reduced by 1.5 GHz (so that at the

biased state LHCP occurs at 130 GHz), an additional margin

has been accounted for in view of experimental uncertainties.

Between these two states (intermediate bias) the orthogonal

linearly polarized state occurs upon reflection when the AR

value exhibits a pole. Hence for this design the change in the

permittivity of the liquid crystals enables the

entire meridian of the Poincare sphere to be covered.

Assuming that the surface is illuminated by a plane wave

linearly polarized at slant angle ξ, the magnitude of the

reflection coefficient, RT, can be calculated from the

magnitude of the measured reflection coefficients for TE and

TM incident waves, RTE, RTM respectively and the slant angle

ξ, using:

{[ ] [ ] } (2)

Fig. 4 also shows the magnitude of the reflection coefficients

for the same orientation angle of the incident signal. The

lowest value is approximately 3 dB less than that shown for

TE incidence in Fig. 3b; this is because for ξ= 45o the wave

vector can be resolved equally between the TM and TE

reflection coefficients of Fig. 3b.

These results in Fig. 4 show that although the phase

difference between the TE and TM reflected waves is exactly

90o at about 130 GHz (Fig. 3a) for the unbiased state, the axial

ratio of the RHCP signal is more than 3.8 dB. This is due to

the magnitude imbalance between TE and TM reflected waves

associated with the increased absorption of the former (Fig.

3b). One option available to compensate for the magnitude of

the imbalance between the TE and TM reflection coefficients,

and hence reduce the AR of the reflected waves, is to vary the

slant angle ξ.

According to Fig. 1, the magnitudes ITE, ITM of the TE and

TM components at incidence are

ITE= I∙sin(ξ) ITM= I∙cos(ξ) (3)

where I is the magnitude of the incident wave. By increasing ξ

it is therefore possible to increase the amplitude of the

impinging TE wave so that upon reflection the desired

amplitude balance is achieved. Since such a modification does

not affect the phase of the reflected TE and TM waves, this

approach can be used to improve the polarization purity of the

CP signal [23]. The computed AR for slant angle ξ= 58o,

which provides an optimum trade-off in terms of AR,

tunability and insertion losses, is superimposed on Fig.

4,

where values of about 0.5 dB are now observed for RHCP

(unbiased state) at 130 GHz. This improvement comes at a

cost of a very slight reduction in the polarization purity of the

orthogonal CP signal because in this case the amplitude of the

reflected TE component is higher than the TM signal (Fig.

3b), leading to a 0.08 dB increase in the value of the AR.

Likewise, higher values of ξ lead to increased thermal losses

because of the increase in the TE absorption. This is depicted

in Fig. 4 where the absorption loss for the unbiased case now

peaks at -3.7 dB.

B. Experimental Testing

The reflection amplitude and phase response of the

electronically tunable polarizer prototype were measured

using an AB mm vector network analyzer [24] in conjunction

with a quasi-optical test bench. The LC filled polarizer was

located at the beam waist in the quasi-optical feed train and

the horn pairs were orientated to measure the responses in the

TE and TM planes. The beam-waist radius at the location of

device under test is approximately 9 mm and the projected

area of the periodic array (75 mm diameter), which is

orientated at 45º, is more than four times this value, therefore

power loss and diffraction effects due to beam truncation can

be neglected. The incident wave front can therefore be

considered as a plane wave of infinite transverse extent. A

photograph of the measured prototype mounted on the quasi-

optical test bench is shown in Fig. 7. The reflection response

of the polarizer was measured by ratioing the detected

reflected power relative to a flat aluminum plate inserted in

the sample holder in the position of the polarizer’s ground

plane. Initially a base level frequency sweep was made

followed by a sweep with the device inserted, with the ratio of

these giving the spectral characteristics of the test sample.

Measurements were made over the frequency range 122-136

GHz using a large sweep setup, which requires retuning of the

system about every 10 GHz.

Fig. 5. Measured axial ratio as a function of frequency for different biasing

voltage values. The tensors shown in eq. (1) have been used for the simulation

of the unbiased (OFF) and biased (ON) states, where ,

, , . Top half of the plot corresponds to RHCP and lower half of the plot to LHCP. Incoming wave linearly polarized at angle

ξ= 58o.

The polarizer was biased using an AC source operating at

10 Hz and with adjustable amplitude up to 89 V. In order to

compare the measured results with those predicted by full-

wave CST simulations, the reflection phase and magnitude

measured for 0 V, 40 V and 89 V are plotted against the

simulated results for the biased and unbiased liquid crystal in

Fig. 3. The good agreement between the simulated and



measured TE and TM plane reflection phases for the unbiased

state (Fig. 3a) confirms the fabrication accuracy of the LC

based polarizer. The measured TM reflection coefficients

agree well with the predicted values (Fig. 3b). However the

measured TE reflection magnitudes are more than 2 dB lower

than the simulated values. Given that the dielectric stack

interacts similarly for both TE and TM incident waves, this is

an indication that higher than predicted Ohmic losses occur on

the dipole array where strong currents are excited for TE

incidence only. It can further be observed that although the

simulated spectral shift for the TE AMC frequency between

biased and unbiased LC states is over 6.5 GHz, the measured

shift is only of the order of 4 GHz; this could be attributed to

the biasing voltage values being limited by the measurement

equipment so that the liquid crystal does not reach its fully

biased state.

Fig. 6. Detailed view of the measured axial ratio and reflection coefficient

magnitude near the operating frequency. Top half of the plot corresponds to RHCP polarization and lower half of the plot to LHCP.

Fig. 7. Photograph of the LC tunable polarizer on the quasi-optical test bench

used for measuring reflection properties. A flat metallic plate located at the polarizer’s ground plane was employed for calibration.

The axial ratio of the polarizer was calculated for slant

angle ξ=58o using the measured data for increasingly higher

values of the biasing voltage amplitude. This is plotted in Fig.

5 and superimposed with the simulated AR for the biased and

unbiased states for comparison. In this plot we have used the

top half to represent axial ratio for right-handed polarization

and the lower half for left-handed polarization assuming

incidence wave polarized at slant angle ξ= +58o (the dual is

valid for incidence wave polarized at slant ξ= -58o). As shown,

for each biasing voltage value there is a frequency point with

axial ratio values in excess of 30 dB; this corresponds to

reflection of a wave linear polarized at an angle supplementary

to that of the incidence signal. At lower frequencies, the axial

ratio approaches the 3 dB RHCP while at higher frequencies

the axial ratio approaches the 3 dB LHCP. In accordance to

Fig. 3, good agreement between the measured and simulated

AR for the unbiased case is obtained with the measured

minimum AR value compromised to a minimum level of

about 1.6 dB due to the amplitude imbalance. Likewise the

discrepancy between the simulated AR for the biased state and

the measured AR for 89 V is attributed to the limited biasing

voltage.

In order to probe further on the performance of the polarizer

at and in the vicinity of the operating frequency, Fig. 6 plots

the axial ratio and reflection coefficient magnitude calculated

as above against the bias voltage. As shown, at 130 GHz, the

reflected wave is RHCP with an axial ratio of 3 dB for 0 V

bias voltage and LHCP with an axial ratio of 3 dB for 89 V

bias voltage. For 40 V bias voltage, the reflected wave is

linearly polarized at ξ= -58o with polarization purity in excess

of 30 dB. This performance is largely maintained within a

bandwidth of about 1 GHz; at frequencies different than the

nominal value of 130 GHz, the polarization purity of either

RHCP/LHCP increases at the expense of the LHCP/RHCP

state.

III. DISCUSSION AND CONCLUSION

An electronically tunable reflection polarizer based on liquid

crystal substrate has been proposed for mm-wave and THz

applications. It is noted that beyond reflection polarizers, the

proposed concept provides a pertinent solution for

electronically reconfigurable phase plates; an observation of

Fig. 4a shows that for one linear polarization (TE) it is

possible to adjust the reflection phase over a range in excess of

180o. By exploiting the scattering properties of arrays with

polarization independent responses [25], it is possible to

provide identical reflection magnitude and phase coefficients

for all incoming wave polarizations. Therefore with the

exception of the unavoidable increased thermal losses, such

structures are the electronically reconfigurable equivalent of a

mechanically moving flat metallic reflector that can shift

along the direction of an incoming beam by an adjustable

distance of up to half wavelength. Arbitrary electrical length

ranges can be achieved if the phase agile mirror is designed to

provide reflection phases over a range of 360o. These surfaces

can thus find application in phase retrieval for mm-wave and

THz radars [26] or tunable interferometers [6].

ACKNOWLEDGMENT

This work was supported by The European Space Agency

(grant A0/1-6169/09/NL/JD). The authors are grateful to Mr

Atsutaka Manabe of Merck KGaA, Darmstadt, Germany for



fruitful discussions and for supplying the liquid crystal

materials.

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